1. Field of the Invention
The invention relates to holographic Light Shaping Diffusers(copyright) (LSDs)(copyright) and, more particularly relates to an LSD formed from a monolithic glass material and to a method of forming a monolithic glass LSD.
2. Background of the Invention
Holographic Light Shaping Diffusers(copyright) (LSDs)(copyright), sometimes known as light shaping homogenizers or simply diffusers, are a type of diffuser used in a variety of illuminating, imaging, and light projecting applications. An LSD is a transparent or translucent structure having an entrance surface, an exit surface, and light shaping structures formed on its entrance surface and/or in its interior. These light shaping structures, sometimes collectively known as speckle (particularly when they are present within the volume of the structure as opposed to only on its surface), are random, disordered, and non-planar microsculpted structures that act as miniature lenses which produce non-discontinuous and smoothly varying changes in the refractive index of the LSD medium. They often are akin in appearance to sponges distributed randomly through the product. These light shaping structures refract light passing through the LSD so that the beam of light emitted from the LSD""s exit surface exhibits a precisely controlled energy distribution along horizontal and vertical axes. LSDs can be used to shape a light beam so that over 90% (and up to 95%-98% of the light beam entering the LSD is directed towards and into contact with a target located downstream of the LSD. An LSD can be made to collect incoming light and either 1) distribute it over a circular area from a fraction of a degree to over 100xc2x0 or 2) send it into an almost unlimited range of elliptical angles. For example, a 0.2xc2x0xc3x9750xc2x0 LSD will produce a line when illuminated by an LED or laser and a 35xc2x0xc3x9790xc2x0 LSD will form a narrow field, high resolution rear projection screen when illuminated by the same light source.
Rather than exploiting a property of monochromatic laser light known as coherence that requires that the finished holographic element be used only at the laser""s wavelength, an LSD operates perfectly in white light. LSDs therefore exhibit a high degree of versatility because they may be employed with light from almost any source, including LEDs, daylight, a tungsten halogen lamp, or an arc lamp.
Two types of LSDs are currently available, namely a xe2x80x9cvolume LSDxe2x80x9d and a xe2x80x9csurface LSDxe2x80x9d. A surface LSD is a surface relief holographic element characterized by the incorporation of light shaping structures (or a computer generated approximation of them) on its surface. A volume LSD is a volumetric holographic element characterized by the incorporation of light shaping structures (or a computer generated approximation of them) within its body and possibly also on its surface. Volume LSDs and surface LSDs are interchangeable in most applications. There are some limited applications, however, in which only volume LSDs can be used, such as applications in which the LSD is submerged in a liquid.
Both volume and surface LSDs typically are produced using a xe2x80x9csub-masterxe2x80x9d that is itself an LSD which contains the holographic surface structures forming the light shaping structures. In the case of a volume LSD, the light shaping structures are recorded in the product structure using standard holographic recording techniques (one or two beam) or a process akin to a printing process. In the case of a surface LSD, the surface structures are embossed or formed in some other way directly onto the surface of the product structure. LSD production using a light shaping structure-bearing master or sub-master is disclosed in U.S. Pat. No. 5,365,354 to Jannson et al. (the ""354 patent), U.S. Pat. No. 5,609,939 to Petersen et al. (the ""939 patent), and U.S. Pat. No. 5,534,386 to Petersen et al. (the ""386 patent). The ""354 patent, the ""386 patent, and the ""939 patent hereby are incorporated by reference for their disclosure of the production of an LSD.
LSDs heretofore were formed solely from plastics such as acrylic or polycarbonate plastics because only these materials were sufficiently deformable (under conditions suitable for interaction with a sub-master) to accept the light shaping structures. Limitations resulting from the physical properties of these plastics restrict the applicable range of LSD operation.
For instance, the plastics from which LSDs are formed typically have a glass transition temperature of below about 150xc2x0 C. and often below about 100xc2x0 C. Conventional plastic LSDs therefore cannot be used in applications in which the LSD may be subjected to sufficient heat to raise the temperature of the LSD to above this glass transition temperature. This heat may be received directly from a light source such as an arc lamp or may be absorbed in the form of UV or infrared radiation. Plastic LSDs therefore generally cannot be used in heat lamps, liquid crystal display projectors, projector lamps, track lighting, or other light sources that generate significant heat near the location of the LSD. Plastic LSDs also are not widely usable with light sources operating in the ultraviolet range or infrared range which emit radiation that is absorbed by the plastic.
Conventional plastic LSDs also are not useable with many UV light sources for the additional reason that the plastic material is a poor transmitter of UV radiation. The typical plastic LSD transmits only about 75% of incoming light of a 365 nm wavelength. Transmission efficiency drops to below about 50% when the incoming light has a wavelength of 350 nm, rendering conventional plastic LSDs ill-suited for use with light sources of less than about 400 nm and effectively useless for light sources of less than about 350 nm. This is a serious limitation of conventional plastic LSDs because many widely-used light sources operate in the UV range, including a mercury laser (365 mn), a triple band laser (355 nm), and a number of excimer lasers (approximately 270 nm).
Another limitation of plastic LSDs is that they cannot be subject to a hot coating operation. It is often desirable to coat a diffuser with a layer of an anti-reflective (AR) coating in order to raise the efficiency of the diffuser. Many coatings, including many AR coatings, can be applied only at temperatures above the glass transition temperature of plastics commonly used in LSDs. Conventional LSDs are not usable with these coatings.
Yet another problem associated with a conventional plastic LSD is that it is difficult or impossible to form a high quality three-dimensional lens on its exit surface. It is desirable in a variety of diffuser applications to place a lens on the exit surface of the diffuser. Conventional plastic LSDs cannot be ground, polished, or molded into high quality lenses. High quality lenses can be produced on the exit surface of an LSD only by laminating or otherwise attaching a Fresnel lens on it. (As is well known in the art, a Fresnel lens is one having a planar or two-dimensional surface that in use creates an effect that is designed to approximate the effect of a three-dimensional curved lens.) Mounting a separate Fresnel lens onto the exit surface of a diffuser is substantially more difficult and expensive than simply grinding or otherwise forming a conventional curved lens on the exit surface and may produce a lower quality lens.
Many of the above-identified disadvantages of a plastic LSD could be avoided if the LSD were to be formed from glass rather than a plastic. However, light shaping structures cannot be embossed on or otherwise recorded in a conventional glass structure during its production process because the high temperatures accompanying formation of conventional glass (on the order of 1,800xc2x0) would destroy the master or sub-master bearing the light shaping structures.
It is therefore a principle object of the invention to provide an LSD that has a wider operating range in terms of temperature and/or wavelength than currently available plastic LSDs.
Another object of the invention is to provide an LSD capable of having a high quality curved lens formed on its exit surface.
Still another object of the invention is to provide a method of making a glass LSD from a monolithic glass material which, when formed, meets some or all of the foregoing objects.
These objects are achieved in a remarkably simple and effective manner by forming an LSD in a glass material which assumes a state during one or more phases of its formation process in which the desired light shaping structures can be embossed on or otherwise recorded in the glass material under conditions hospitable to the master or sub-master. Preferably, the light shaping structures are produced during formation of a so-called xe2x80x9csol-gelxe2x80x9d glass either by an imaging technique such as a photolithographic imaging technique (thereby forming a volume LSD) or by an embossing technique (thereby forming a surface LSD).
In the case of castable sol-gel glasses, volume LSDs can be produced by (1) applying a photosensitizer to the sol-gel material during formation of the monolithic oxide glass, thereby rendering the monolithic oxide glass photosensitive, and (2) exposing selected portions of the photosensitive monolithic oxide glass to light through a photomask containing light shaping structures to form a metal oxide in the exposed portions of the mask which binds irreversibly with the monolithic oxide glass, thereby producing a volume LSD. The applying step may comprise mixing the photosensitizer in stoichiometric amounts with sol-gel precursors during preparation of the solution or depositing the photosensitizer onto the porous glass after the aging step.
Surface LSDs can be produced from castable sol-gel glasses simply by casting the solution in a plastic mold bearing the light shaping structures on an inner surface thereof so that the light shaping structures are embossed on the sol-gel material during the sol-to-gel transition process.
Both volume LSDs and surface LSDs also can be produced from coatable sol-gel glasses by coating a layer of the sol-gel solution onto a substrate to produce a film layer on the substrate, causing the film layer to undergo a sol-to-gel transition, recording light shaping structures in at least a portion of the film layer, and aging the gel to form a porous glass. The final step in the preferred process is to heat treat the glass to its sintering temperature to produce a non-porous glass. In the case of a volume LSD, a photosensitizer is added to the sol-gel solution and the recording step comprises (1) placing a mask over the film layer (the mask bearing the light shaping structures) and then (2) exposing the mask to light to form a metal oxide in exposed portions of the film layer which binds irreversibly to the glass in the film layer. A surface LSD can be formed by the same process if the thickness of the photosensitized layer is approximately the same as the depth of relief on the master used to produce a true surface relief LSD. In the case of a surface LSD, the recording step comprises pressing a plastic master bearing the light shaping structures into contact with the film layer.